The major swoop saw around 75 police officers from Dyfed-Powys Police execute a warrant at Leighton Arches Caravan Park, near Welshpool, at 7am this morning.
The suspects were arrested on suspicion of being involved in the supply of heroin and cocaine.
Teams were expected to remain at the site into the early evening with a cordon in place.
Powys Commander, Superintendent Ifan Charles, said: “The action we’re taking today is part of our ongoing commitment to tackle illegal drug dealing in Welshpool and beyond, and is the result of months of hard work.
“We take the concerns of our communities seriously, and I hope the residents of Welshpool are reassured by this activity.
“Teams of specialists remain at the site of the warrant, working meticulously to ensure any evidence of wrongdoing is secured.
“I urge anyone with information or any concerns about illegal drug dealing to raise them with police so we can take action.”
The men, aged 31 and 23, along with the teenager, remained in police custody on Wednesday afternoon.
Anyone with information that could help the investigation is urged to contact Dyfed-Powys Police either online by email at firstname.lastname@example.org, or by calling 101. If you are deaf, hard of hearing, or speech impaired text the non-emergency number on 07811 311 908.
If you would prefer not to contact police, report information anonymously to the independent charity Crimestoppers, by calling 0800 555 111.